1. Field of the Invention
The present invention relates to devices intended for separating both solid and pulp-like materials by size range. More specifically, the invention concerns the design of vibrating screens.
The invention may prove most advantageous in mining, construction, and metallurgical industries for separating ores, building materials, and metallurgical burdens respectively.
2. Prior Art
Separation of particles by size range is carried out in such devices in the course of shifting a layer over the screen. Fine fractions of the material shift within the layer under the action of vibrations till the contact with the screen and upon having reached the screen pass therethrough.
The practice of separating free-flowing and pulp-like materials by size range on vibrating screens indicates that these screens have to meet the basic requirements that the screen must ensure high separation efficiency and possess high specific throughput, which becomes possible with a high speed of fine fractions moving through the material layer till the contact with the screen, and rapid passage of those particles of fine fractions which have reached the screen, through the screen openings. At the same time, the screen must have a high degree of vibration damping, i.e. transmit the minimum amount of dynamic forces to the base.
At present, in mining, construction, and metallurgical industries, screens are utilized wherein vibration amplitudes are equal at each point of the screen, and vibration forces are directed at the same angle to the screen plane, i.e. a uniform vibration field acts along the screen length. In numerous cases, the layer height in these screens is such that during the time of movement of the material layer over the screen, fine fractions do not have enough time to go down till the contact with the screen, thereby the separation efficiency gets decreased. In order to achieve the high efficiency of separation in these screens, it is desired that the material layer being shifted be of a small height. However, in so doing the specific throughput of the screen is decreased. Thus, in such screens the requirement of ensuring the high separation efficiency simultaneously with the high specific throughput, is not met.
It is well known that the speed of fine fractions shifting through the layer of material becomes increased in the case where vibration forces act on the layer, said forces inducing pure shear stresses within the layer. When the vibration forces induce simultaneously shearing stresses and tensile and compressive stresses within the layer, the fine fractions move through the layer till the contact with the screen at a considerably lower speed. It is also known that the fine fractions, which have reached contact with the screen, pass through this screen with the highest rate in the case where an angle of throwing-up the layer above the screen is close to the right angle. Thus, to increase the speed of the fine fractions moving through the layer towards the screen, it is necessary that the screen be vibrating in the scattering plane, while to ensure the best passage of particles through the screen, the latter must vibrate in the plane perpendicular to the scattering plane.
In conventional vibrating screens, the vibration forces are directed at a certain acute angle to the scattering plane, i.e. the conditions of vibrations are not optimum either for the process of movement of the fine fractions through the layer till the contact with the screen, or for the process of passage of the particles which have reached the screen through the openings thereof.
In some cases there are applied vibrating screens possessing a vibration field which is non-uniform along the screen length, where the amplitude of vibrations and the direction of vibration forces along the screen length change in disordered manner. It is obvious that such screens do not meet the requirements.
In certain screens known in the art, the vibration field is non-uniform, and the amplitude of vibrations and the direction of the vibration force change in ordered manner along the screen length from the charging end towards the discharging one. These screens are known as the Baum screens (see Spravochnik po obogashcheniyu rud, Moscow, "Nedra", 1972, s.68). As a rule, the Baum screen comprises a box with a screen mounted thereon, supports constructed as dampers and levers inclined towards the central axis of inertia of said box, and an oscillation driver constructed in the form of a crank and connecting rod mechanism fixed on a frame and connected with said box by means of connecting rods.
Although the Baum screen does not completely meet the requirements of increasing the separation efficiency and specific throughput, since its ordered non-uniform vibration field is not optimum, its utilization allows this problem to be solved to a certain extent.
However, the Baum screen is not internally balanced and transmits the loads acting within the drive to the base, thereby making its application as highly productive industrial models impossible.